Electro-optic displays

ABSTRACT

An electro optic display ( 10 ) provides an electrode configuration adjacent each pixel which allows a non-uniform electrical field to be applied across the pixel ( 22 ) so that the optical output varies in the direction transverse to the pixel thickness. This allows only part of the pixel to be turned ON, or for shading within the pixel, depending on the characteristics of the electro-optic material. In a preferred embodiment the row and column electrodes ( 14, 17 ) are each made up of a group of conductive tracks ( 12, 18 ) connected into groups by impedance elements ( 13, 19 ). Voltage ramps are applied across the electrodes via input electrodes  15  and  20 . A multiphase drive scheme is described for ferro-electric and similar materials in which a number of different voltage ramps are applied to a row in succession, and at each phase the appropriate ramps are applied simultaneously to the column electrodes to build up the required pixel shape over a number of phases.

The above-identified application is a divisional of U.S. patentapplication Ser. No. 08/930,819 filed Oct. 8, 1997 which is the U.S.National Stage Appl. of PCI International Appl. No. PCT/GB96/01009 filedApr. 29, 1996.

FIELD OF THE INVENTION

This invention relates to electro-optic displays.

BACKGROUND OF THE INVENTION

Many computer displays have a limited physical resolution—typically70-100 dots per inch (2.76 to 3.94 dots per mm). Since the display iscomposed of an array of rectangular pixels, each of which is either ONor OFF, the edges of text, lines, etc. that are displayed may often havea jagged “staircase” effect which is visually disturbing.

There have been several attempts to solve this problem in a variety ofways e.g. by grey scale rendering or anti-aliasing. Some displaytechnologies such as Twisted Nematic Liquid Crystal Displays (TN LCDs),cathode ray tubes, etc., allow the intensity of the whole area of apixel to be varied. In these types of display, the intensity of eachpixel may be selected in proportion to the area of the pixel that shouldbe ON. Whilst this can reduce the visually disturbing staircase effect,it can make edges in the displayed image appear blurred, especially whenviewed from close to the display.

Other display technologies do not allow a range of intensities over thewhole area of the pixel display, in which case a range of intensitiesmay be simulated by rapidly turning the pixel ON and OFF, sufficientlyfast so that the eye sees the average intensity. This may be used withSuper Twisted Nematic (STN) LCDs. Alternatively, each pixel may bedivided into sub pixels, and a varying number turned on according to thedesired intensity. The display must then be viewed from such a distancethat the eye cannot resolve the sub pixels, or some optical blurringintroduced to average out the intensity over the whole pixel. An exampleof this type of technique is described in JP-A-3142260, where each pixelis effectively divided into four sub pixel slices. The image to bedisplayed is analysed and two-bit pixel data is added to each pixel toturn on selected sub pixel slices during a pixel sub-scanning period.This allows a range of intensities to be displayed by varying the areaof the pixel that is ON, in four discrete steps. However, by its naturethis system is only capable of modulating the pixel output slice-wiseand in many instances this will not give good smoothing, particularlywhere the edge to be smoothed is nearly perpendicular to the slicedirection of the pixels.

A similar technique is disclosed in U.S. Pat. No. 4,824,218, whichrelates principally to Ferroelectric LCDs. Here a variable width portionof a pixel is turned on by driving a potential gradient across the widthof the pixel by means of metal electrodes running along the edges ofresistive transparent column electrodes. To allow the complete area ofthe pixel to be driven, whilst preventing crosstalk (i.e.unintentionally affecting other pixels in the same row), and to avoid awasted area of the pixel nearest the “reference” metal electrode, thepixel is driven in two phases, swapping the role of the two metalelectrodes between “reference” and “data”. This technique relies on thefact that a Ferroelectric Liquid Crystal (FLC) material stores its stateand can be written to again, adding to the area that has already beenturned ON, which is not true for all bistable materials. This schemealso requires a blanking pulse to clear the whole pixel before the twowriting phases. U.S. Pat. No. 4,824,218 also refers to an extension ofthis technique in which the display has row and column transparentresistive electrodes each with metal electrodes on either side. A fourfield drive scheme is described in which the display is scanned fourtimes, following a blanking pulse, to make up a frame. As before themetal electrodes swap roles between data and reference electrodes.Because alternate electrodes are set to a fixed reference thisarrangement does not allow great flexibility in the creation of subpixel shapes.

SUMMARY OF THE INVENTION

Accordingly, in one aspect, this invention provides a displaycomprising:

a first substrate having thereon a plurality of row electrode means,

a second substrate having thereon a plurality of column electrode means,

a layer of electro-optic material disposed between said first and saidsecond substrate,

row drive means associated with said first substrate for applying arespective selected voltage profile across each row electrode means in adirection transverse to the thickness of said layer,

column drive means associated with said second substrate for applying arespective selected voltage profile across each column electrode meansin a direction transverse to the thickness of said layer,

whereby the electrical field in each pixel in the direction transverseto the thickness of the electro-optic layer may be selected to provide anon-uniform optical output.

In a particularly preferred arrangement, each of said column electrodemeans and said row electrode means comprises a group of generallyparallel conductive tracks.

We have found that the multiple track architecture for each of the rowand column electrodes provides important and unexpected advantages whenused in conjunction with drive means which apply a selected voltageprofile across each of the groups of tracks making up an electrode. Inthis way the magnitude of the electrical field across the pixel may bevaried in a direction transverse to the thickness of the electro-opticlayer to provide a non-uniform optical output across the pixel. Bycontrast to the arrangements of U.S. Pat. No. 4,824,218, which useresistive electrodes driven by metal electrodes to either side, themultiple conductive tracks of the present invention may be driven byelectrical contacts well away from the image area, thus considerablyimproving the aperture ratio of the display. Also, the previousarrangement of U.S. Pat. No. 4,824,218 requires accurate alignment offine metal electrodes with each of the transparent resistive electrodes,whereas in the present invention the conductive tracks in each group maybe coupled together by a resistive element in contact with the endregions of the conductive tracks to one side of the display, and aninput electrode provided at each end of the resistive element. Here theaccuracy of alignment of the tracks and the resistive elements is not soimportant as the resolution of the conductive tracks may be effectivelydecoupled from that of the resistive elements.

Thus, in one embodiment, the resistive elements may be formed on thefirst and second substrates, in electrical contact with the respectiveconductive tracks. Alternatively, the row and column resistive elementsmay be formed on separate substrates which are then placed in contactwith the first and second substrates.

For both the rows and the columns, the series of resistive elementsdriving the groups of conductive tracks may be replaced by a singleresistive element in electrical contact with a substantial proportionof, or all the conductive tracks making up the complete set ofrow/column electrodes, with the drive means including an input electrodemeans between each group of conductive tracks.

Preferably, said drive means includes means for applying an adjustablevoltage across each of said resistive elements, so that the voltageprofile across the group of conductive tracks is a ramp of positive,negative or zero slope. Instead of resistance coupling, the drives tothe groups of tracks may be inductively or capacitively coupled.

By the above arrangements, the electrical field may be configured togenerate a wide range of different non-uniform outputs of selected shapefor a pixel, to turn on an arbitrary portion of the pixel, so that therequired edge of the line portion of the text character etc. ismaintained within the area of the pixel.

In one preferred drive scheme, each of a set of predetermined voltageprofiles is applied across a row electrode means in successive phases,and the columns driven in parallel with the required voltage profiles.In this way a broad range of pixel shapes may be provided in either asingle write (i.e. just one of the phases) or a multiple write where thepixel output is incrementally rendered. It will be appreciated that thisdrive scheme could be modified so that the multiple successive phasesare applied to the columns whilst the rows are driven in parallel.

The electro-optic material may have a steep or “fast” electro-opticcurve, i.e. where the electro-optic effect switches state abruptly at aparticular threshold voltage, so that the optical output at a particularpoint within the pixel will vary between two generally discrete levels,dependent on whether the field strength at that point is above or belowthe threshold voltage. In general the boundary between ON and OFFregions in the pixel will be determined by an equi-potential line on thenotional voltage surface within the pixel of magnitude corresponding tosaid threshold voltage. Alternatively, the electro-optic material mayhave a shallow or “slow” electro-optic curve, with the output varyingcontinuously between ON and OFF through grey levels, so that the opticaloutput may be shaded across the pixel, generally in accordance with themagnitude of the electrical field.

We have found that the technique of providing modulation of the voltagefield in a direction transverse to the thickness of the electro-opticlayer may be extended to other forms of display, again to provide anon-uniform optical output, with remarkable results.

Thus, in another aspect this invention provides a display comprising:

first and second substrate means provided to either side of a layer ofelectro-optic material,

the first substrate having thereon at least one electrically resistivesurface,

electrode means for applying selected voltages to respective points orregions across said resistive surface, and

drive means for applying selected voltages to said electrodes to provideacross a pixel an electrical field whose magnitude varies in at leastone direction transverse to the thickness of the layer of electro-opticmaterial, thereby to provide a non-uniform optical output across saidpixel.

The first substrate may have an array of discrete electrically resistivesurface elements each defining a pixel, with each surface element havingthree or more electrodes attached thereto. Thus, three or more selectedvoltages may be applied to spaced points or regions of said discreteresistive layer element, for example a respective voltage at each cornerof a rectangular pixel. In this arrangement, each pixel effectivelyinterpolates the four corner voltage values around the pixel and, for anelectro-optic material with a steep curve, at each point within thepixel where the voltage passes the threshold value, the optical outputchanges.

We have also found that the ability to vary the optical output across apixel instead of uniformly over the whole pixel means that we canprovide displays which provide an effect similar to the half-tone typeof process. In one arrangement the image is made up of pixels eachcontaining a dot of controllably variable size from zero to fullyfilling the pixel, or even extending beyond its nominal boundary. Thus adisplay may comprise an electrode array of resistive electrodes with aplurality of connection points whereby an image may be created by adisplay of variable size dots.

The electro-optic material may comprise a liquid crystal material suchas twisted nematic, supertwist nematic, polymer dispersed liquid crystal(PDLC), or ferro-electric materials. Alternatively it may comprise otherelectro-optic materials or devices, such as field emissive devices.

In operation of a typical example, the image to be displayed is analysedto determine those pixels where some form of intra-pixel variation isrequired, for example where the pixel is lying on an edge of the imageor where grey-scaling or half toning is required. For each such pixel,the display driver determines the desired boundary lines between ON andOFF, and the electrical field intensity required, and then selectssuitable voltages to be applied to the pixel to generate the requiredvariation or level of electrical field across the pixel.

Whilst the invention has been defined above, it extends to any inventivecombination of the features set out above or in the followingdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be performed in various ways and, by way of exampleonly, certain embodiments thereof will now be described with referenceto the accompanying drawings, in which:

FIG. 1 illustrates schematically a first embodiment of liquid crystaldisplay device in accordance with the invention;

FIG. 2 shows typical voltage profiles applied across a pixel and thenon-uniform output obtainable in the embodiment of FIG. 1, with FIGS.2(a) and (b) showing the voltage profiles applied across the rowelectrode and the column respectively, FIG. 2(c) showing the resultantelectrical field set up across the pixel, and FIG. 2(d) showing thenon-uniform optical output obtained;

FIG. 3 illustrates an alternative driving electrode scheme for theembodiment of FIG. 1;

FIG. 4 illustrates schematically a first type of drive scheme fordriving the embodiment of FIG. 1;

FIG. 5 illustrates a control system for controlling embodiment of FIG.1;

FIG. 6 illustrates schematically the pixel electrode scheme for a secondembodiment of liquid crystal display device in accordance with theinvention;

FIG. 7 shows in FIG. 7(a) a typical voltage surface effected in theembodiment of FIG. 6, and in FIG. 7(b) the resultant non-uniform outputobtained;

FIG. 8 illustrates schematically the pixel electrode scheme for a thirdembodiment of liquid crystal display in accordance with the invention;

FIG. 9 shows in FIG. 9(a) a typical voltage surface effected in theembodiment of FIG. 8, and in FIG. 9(b) the resultant non-uniform outputobtained;

FIG. 10 illustrates schematically a fourth embodiment of liquid crystaldisplay in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the first embodiment of LCD display 10 of thisinvention is illustrated. For clarity, the upper glass or polymer coverlayer has been omitted, together with the usual polarising, barrier andretardation layers, but these are of generally conventional design. Thedisplay 10 comprises a lower glass or polymer substrate 11 on which aseries of electrically conductive tracks 12 is formed, for example froma high conductivity Indium Tin Oxide (ITO) layer.

The conductive tracks 12 are connected together in groups of four by aresistive pad 13 to one side of the display, each group defining acomposite row electrode 14. A selected voltage ramp may be appliedacross the resistive pad 13 by means of input electrodes 15. The voltageapplied to each electrode is independently selectable to allow a rangeof voltage profiles to be applied. The effect of applying a voltage rampacross the resistive pad is to set each of the conductive tracks 12making up the composite electrode to a predetermined adjustable voltageand effectively to set up a potential gradient across the lower surfaceof each of the pixels contained in the row, in the plane of theelectrode. A polymer alignment layer (not shown) overlies the conductivetracks and serves to align the adjacent liquid crystal layer 16.Adjacent the upper surface of the liquid crystal layer 16 there isprovided a further polymer alignment layer and a set of composite columnelectrodes 17 each comprising a group of conductive tracks 18interconnected by resistive pads 19, and similar in construction andgrouping to the row electrodes. A voltage ramp may be applied across thecolumn electrode by means of input electrodes 20 at opposite ends ofeach of the resistive pads. In this arrangement a pixel 22 is defined ateach crossing of a composite row electrode 14 by a composite columnelectrode 17.

As to be described the illustrated arrangement allows the orthogonalvoltage profiles across the upper and lower surfaces bounding a pixel tobe independently adjusted to apply a compound voltage profile to thepixel. The optical output at a particular point in the pixel will dependon whether the compound voltage at that point is or, for some materials,has been above the threshold voltage for that material. In this way avariety of complex geometrical shapes may be written within the pixel bysuitable control of the voltage ramps applied to the relevant compositerow and column electrodes.

In the following, for convenience we refer to the electrodes applyingcontinuously voltage surfaces across the upper and lower pixel faces butit will be appreciated that, due to the stepped nature of the compositeelectrodes, the voltage surfaces in practice may be stepped. It isassumed in this example, that the electro-optic material switches at 5voltage units. FIG. 2(a) shows the voltage profile applied across thelower surface of the pixel by means of the relevant composite rowelectrode 14, by applying 10 voltage units to the upper row inputelectrode 15′ and 15 voltage units to the lower row input electrode 15″.Likewise FIG. 2(b) shows the voltage profile applied to the uppersurface by means of the composite column electrode 17, by applying 10voltage units to the left column electrode 20′ and 0 voltage units tothe right column electrode 20″. FIG. 2(c) shows the resultant fieldacross the pixel corresponding to the difference between the two voltagesurfaces, and FIG. 2(d) gives the resultant optical output. In practice,due to the stepped nature of the voltage surfaces the dividing line maynot be straight but this is unlikely to be perceptible. Naturally, theresolution may be improved by increasing the number of electrodes in agroup.

A wide range of sub pixel shapes may thus be generated by suitableselection of the adjustable voltages (four in all) applied to thetop/bottom input electrodes of the resistive pad 13 on the relevantcomposite row electrode 14,and the left/right input electrodes of theresistive patch on the relevant composite column electrode 17. It shouldbe noted that selected sub portions of the pixel may be set or unset asrequired.

A modified electrode structure is shown in FIG. 3. Here a singleresistive pad 26 is in electrical contact with the resistive tracks 12making up a number of composite row electrodes, and may interconnectwith all the row tracks 12. Between each group of tracks making up theelectrode is an input electrode 28 so that the required voltage rampsmay be set up across each of the composite row electrodes either singly,or in combination with a number of adjacent rows. Also, the whole, or avariable sized portion of the display may be driven, e.g. by driving theupper electrode of the first row and the lower electrode of the lastrow.

Various drive schemes are possible and two will now be described withreference to FIGS. 4 and 5. The first scheme is for a LCD material suchas Ferroelectric LCD and similar materials which can be selectively setand unset. Firstly a blanking or erasure pulse is applied to a compositerow electrode 14 to clear the pixels in the row. The row is then writtenby a multi-phase write, in which a finite set of voltage ramps isapplied across the upper and lower input electrodes 15′, 15″ on theresistive pad, for example “High” (both set high), “Low” (both set low),positive slope (upper set high, lower set low) and negative slope (upperset low, lower set high).

At each phase, the composite column electrodes 17 that would requirethat particular “vertical” ramp on their lower surface have theappropriate respective horizontal ramps set on each of the inputelectrodes 20′, 20″ on each of the column resistive pads 19. This allowscomplex shape pixels to be set up as the union or difference of theprimitive shapes set up by any particular combination of row/columnramps. The maximum amplitude of the two ramps is set below half thethreshold voltage of the material to prevent cross talk. FIG. 4 showshow the shapes in the pixels in a given row may be built up insuccessive phases. Thus in phase 1 a steep positive ramp is applied tothe composite row electrode 14. Simultaneously via the respectivecomposite column electrodes 17, a positive ramp is applied to the firstpixel, a low ramp is applied to the second, a negative slope to thethird and fourth, and a high is applied to the fifth, to give the firstphase results as shown. In the second phase a gentle positive slope isapplied to the composite row electrode 14. Simultaneously, low ramps areapplied to the first, third and fourth pixels (resulting in no change),a negative slope is applied to the second pixel and a positive slopeapplied to the fourth. This writes part of the second pixel and modifiesthe fifth pixel. In the next phase a gentle negative slope is applied tothe composite row electrode. Simultaneously, low ramps are applied tothe first second and fifth pixels, a positive slope is applied to thethird pixel and a high slope applied to the fourth pixel.

The display may be written with all the phases for a row completedbefore the next row is selected, as suggested by FIG. 4, or each phasemay be written on a row by row basis, or a variety of interleavingschemes could be used, to maximise clarity and immediacy of theinformation displayed.

Referring to FIG. 5, in this example the image data to be presented onthe display 10 is processed using known processing techniques todetermine the boundary pixels at the edge of text or images etc., wherejagged edges would be visually unacceptable.

The image processor 24 incorporates a look up table 26 which indicatesthe pixel shapes obtainable and the voltage ramps required to producethese shapes. The image processor 24 then selects the appropriatevoltage ramps for the driving phases to match the shapes required andcontrols the row and column drives 27, 28 accordingly.

In a preferred drive scheme for some materials such as StabilisedCholesteric Liquid Crystals the previous state is wiped out on writingand thus no separate blanking pulse is applied. Here a modified approachis required. As before, a number of voltage ramps are set up on the rowelectrode in successive phases, but a drive ramp is applied to thecolumn electrode of a particular pixel in only one row phase, thatcolumn being quiescent (i.e. at a level which will not induceswitching), for the other phases.

This scheme give straight edges (or approximating thereto) to theintra-pixel boundary. For situations where the pixels may appear quitelarge, e.g. for electronic signs, projection displays, or head mounteddisplays, it is desirable to have more control over the edge shape ofthe boundary of the ON region of the pixel e.g. to provide generallysmooth curves.

In the embodiment of FIG. 6, the output of each pixel may be variednon-uniformly by a multi-point electrode array provided here in the rearstructure of the display, with a layer adjacent the upper surface of theliquid crystal layer providing a common ground plane. For clarity, theusual polymer alignment, polarising, barrier and retardation layers arenot shown.

The display 30 includes a multi-layer rear structure 32 which defines anarray of resistive surface elements 34, one adjacent each pixel, aliquid crystal layer 36, and a transparent common conductive layer 38serving as a voltage ground plane adjacent the upper surface of theliquid crystal layer 36. Each resistive surface element 34 is energisedby four control electrodes 40 or control points, one adjacent eachcorner of the surface element 34 respectively. The notional voltagesurface generated over a pixel will depend on the levels of the voltagesapplied at the four corners tending to a smooth surface interpolatingthe four corner voltages, and can be adjusted by suitable selection ofthe four corner voltages, as illustrated in FIG. 7(a). The resultantnon-uniform output generated by the voltage surface of FIG. 7(a) in anelectro-optic material having a steep electro-optic effect is shown inFIG. 7(b), and generally provides a curved boundary.

Again a variety of drive schemes are possible depending on the number ofpixels in the array. For a low number of pixels there may be sufficientspace between or under the resistive surface elements to drive eachcorner voltage for all the pixels independently.

Likewise, the image to be displayed may be analysed by an imageprocessor as described in relation to FIG. 5, to determine those pixelsrequiring non-uniform outputs, the shapes required, and the appropriatevoltage combinations that will achieve this in a single phase or inmultiple phases as required. The appropriate signals are then suppliedto the row and column drives.

Referring now to FIG. 8, the third embodiment is similar in manyrespects to that of FIG. 6, as regards the general structure. In thisembodiment, the display includes a rear multi-layer structure 46 onwhich is provided a continuous resistive layer 48, which is energised byan array of control electrodes 50 spaced closely across the layer. Theliquid crystal layer is bounded on its other side by a transparentcommon conductive layer 52 serving as a ground plane. In use, thevoltages applied to the electrodes may be selected to give a voltagesurface including at each electrode location a voltage peak of selectedmagnitude as shown in FIG. 9(a). Where the liquid crystal material has asteep electro-optic curve, these peaks translate into dots of variableradius dependent on the radius of the peak at the threshold voltage. Thedrive voltages to the electrodes are therefore selected to give therequired dot size, and a typical resultant image is shown in FIG. 9(b).This embodiment allows images to be displayed using a half-tone type ofprocess. Dots of variable size may be generated and merged if requiredto give the final image. In this embodiment, each control electrodeeffectively defines an unbounded pixel which, depending on the voltageapplied, may encroach into or merge with adjacent pixels.

The embodiments of FIGS. 6 and 7 may both use the multi-layer rearstructure of the display to incorporate the electrode configurationsrequired and this may be particularly suitable for displays usingpolymer dispersed liquid crystal material.

FIG. 10 shows a further embodiment of this invention, in whichrespective voltage ramps may be set up across the upper and lowersurfaces of a pixel by setting two adjustable voltages on the relevantrow electrode and two further adjustable voltages on the relevant columnelectrode, thus providing a similar effect to the embodiment of FIG. 1,and capable of using similar drive schemes. In this embodiment, theupper glass or polymer cover layer has been omitted, together with theusual polarising, barrier and retardation layers, but these are ofgenerally conventional design. The display 70 comprises a lower glass orpolymer substrate 77 on which a series of electrically resistive tracks74 is formed, for example from a high resistivity linearly conductiveIndium Tin Oxide (ITO) layer. To either side of each resistive track 74is deposited a low resistivity conductor 76 in good electrical contacttherewith, for example of chromium or gold, so that a generally linearlyvarying electrical field may be set up across the surface of theresistive track 74. In this example each resistive track 74 has its ownpair of conductors 76, although in other arrangements, adjacentresistive tracks 74 may share the same conductor. A polymer alignmentlayer 78 overlies the tracks 74 and conductors 76 and serves to alignthe adjacent liquid crystal layer 80. Adjacent the upper surface of theliquid crystal layer 80 there is provided an orthogonal series ofresistive tracks 82 and associated conductors 84 deposited on a coverlayer (not shown). Only one upper resistive track 82 is shown, but itwill be appreciated that in practice there will be many individuallyaddressable parallel tracks 76 and 84 in both directions, defining rowsand columns of pixels, e.g. at 86.

In use, the voltage profiles across the tracks 82 and 74 above and belowa pixel 86 are selected to give a notional voltage surface across thepixel which generates the required electro-optic output. Thus, a typicalvoltage profile applied to the upper track 82 is an inclined ramp orplane bridging the voltage levels applied to the conductors 76.

Likewise, a typical profile applied to the lower track, is an inclinedplane between the voltage levels applied to the conductors 76.

The ramps, and their combined effects are therefore similar to thosedescribed in connection with FIGS. 2(a) to 2(d). The various driveschemes mentioned above may be used to drive this embodiment.

The above embodiments may be used with a wide range of combinations ofelectro-optic materials and addressing techniques. Thus the inventionmay also be embodied in Passive Matrix Twisted Nematic (TN) LCDS,Passive Matrix Supertwist Nematic (STN) LCDs, Active Matrix TN LCDs,Stabilised Cholesteric Liquid Crystal Devices, Passive MatrixFerroelectric and Field Emissive Devices.

What is claimed is:
 1. A display comprising, first and second substratemeans provided to either side of a layer of electro-optic material, thefirst substrate means having thereon at least one electrically resistivesurface, electrode means for applying selected voltages to respectivepoints across said resistive surface, said electrode means comprising anarray of point electrodes, drive means for applying selected voltages tosaid electrode means to provide across a pixel an electrical field whosemagnitude varies in at least one direction transverse to the thicknessof the layer of electro-optic material, thereby to provide a non-uniformoptical output across said pixel, wherein said voltage at each pointelectrode is independently controllable.
 2. A display according to claim1, wherein said first substrate means has thereon an array of discreteelectrically resistive surface elements each defining a pixel, eachsurface element associated therewith electrode means comprising a groupof at least three electrodes.
 3. A display according to claim 2, whereineach of said resistive surface elements is generally rectangular, withthe electrodes of the group located in the corner regions thereof.
 4. Adisplay comprising, first and second substrate means provided to eitherside of a layer of electro-optic material, the first substrate meanshaving thereon at least one electrically resistive surface, electrodemeans for applying selected voltages to respective points or regionsacross said resistive surface, drive means for applying selectedvoltages to said electrode means to provide across a pixel an electricalfield whose magnitude varies in at least one direction transverse to thethickness of the layer of electro-optic material, thereby to provide anon-uniform optical output across said pixel, wherein said electricallyresistive surface is a continuous electrically resistive surface, andsaid electrode means comprises a plurality of electrodes for applyingselected voltages to respective points or regions on said resistivesurface, and wherein said plurality of electrodes comprises an array ofpoint electrodes for applying selected voltages to the respective pointsat spaced locations across the electrically resistive surface, whereinsaid voltage at each point electrode is independently controllable.
 5. Adisplay comprising, first and second substrates provided to either sideof a layer of electro-optic material, the first substrate having thereonat least one electrically resistive surface, a plurality of electrodesfor applying selected voltages to respective points or regions acrosssaid resistive surface, a voltage source for applying selected voltagesto selected electrodes of said plurality of electrodes to provide acrossa pixel an electrical field whose magnitude varies in at least onedirection transverse to the thickness of the layer of electro-opticmaterial, thereby to provide a non-uniform optical output across saidpixel, and wherein said electrically resistive surface is a continuouselectrically resistive surface, and said plurality of electrodescomprises an array of point electrodes for applying selected voltages tothe respective points at spaced locations across the electric resistivesurface, wherein said voltage at each point electrode is independentlycontrollable.
 6. A method of operating a display comprising first andsecond substrates provided to either side of a layer of electro-opticmaterial, the first substrate having thereon at least one electricallyresistive surface, and the display further comprising a plurality ofelectrodes for applying selected voltages to respective points orregions across said resistive surface, wherein an array of pixels isdefined for the display, the method comprising: applying selectedvoltages to selected electrodes of said plurality of electrodes toprovide across one or more pixels an electrical field whose magnitudevaries in at least one direction transverse to the thickness of thelayer of electro-optic material, thereby providing a non-uniform opticaloutput across said pixel, wherein said electrically resistive surface issubstantially continuous and said plurality of electrodes comprises anarray of point electrodes for applying selected voltages to therespective points at spaced locations across the electrically resistivesurface, wherein said voltage at each point electrode is independentlycontrollable, wherein applying selected voltages to selected electrodescomprises providing peak voltages selected for each selected electrode;and providing an area of optical output around each selected electrodedetermined by the selected voltage applied thereto.
 7. A displaycomprising: first and second substrates provided to either side of alayer of electro-optic material, the first substrate having thereon atleast one resistive surface; an array of point electrodes forselectively applying voltages to points on the resistive surface; andvoltage drivers for applying selected voltages to the point electrodesof the array to provide voltages varying between point electrodes on theresistive surface and hence electrical fields between correspondingpoints on the first and second substrates and the optical output of theelectro-optic material vary in accordance with the voltages varyingbetween point electrodes, wherein said voltage at each point electrodeis independently controllable.
 8. A display as claimed in claim 7,wherein the resistive surface comprises an array of resistive cells. 9.A display as claimed in claim 8, wherein each of said resistive cellshas four sides, and wherein at least three corners of each resistivecell contains a point electrode from the array of point electrodes. 10.A display as claimed in claim 7, wherein the resistive surface iscontinuously resistive between successive point electrodes.
 11. A methodof driving a display having first and second substrates provided toeither side of a layer of electro-optic material, the first substratehaving thereon at least one resistive surface, an array of pointelectrodes for selectively applying voltages to points on the resistivesurface; and voltage drivers for applying selected voltages to the pointelectrodes of the array, the method comprising: selectively applyingdifferent voltages to at least two different point electrodes of thearray to establish voltage gradients between said two different pointelectrodes across the resistive surface, wherein said voltage at eachpoint electrode is independently controllable; thereby driving thedisplay such that a threshold voltage for the layer of electro-opticmaterial is exceeded at a point between the two different pointelectrodes.